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Tandem Reactions via Barton Esters with Intermolecular Addition and Vinyl Radical Substitution onto Indole Robert Coyle, Patrick McArdle, and Fawaz Aldabbagh J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo5008543 • Publication Date (Web): 16 May 2014 Downloaded from http://pubs.acs.org on May 26, 2014
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Tandem Reactions via Barton Esters with Intermolecular Addition and Vinyl Radical Substitution onto Indole
Robert Coyle, Patrick McArdle and Fawaz Aldabbagh* School of Chemistry, National University of Ireland Galway, University Road, Galway, Ireland ∗ Corresponding author. Tel.: +353 (0) 91 493120; fax: +353 (0) 91 49557 e-mail: [email protected] (F. Aldabbagh)
ABSTRACT: A one-pot initiator-free Barton ester decomposition, tandem radical addition onto alkyl propiolates or phenylacetylene with aromatic substitution of the resultant vinyl radical allows convenient access to new 9-substituted 6,7-dihydropyrido[1,2-a]indoles. Propyl radical cyclizations compete when forming the expanded 7,8-dihydro-6H-azepino[1,2-a]indole system. 2-Thiopyridinyl S-radical is incorporated into aromatic adducts when using unsubstituted indole-1-alkanoic acid precursors. X-ray crystallography on substitution products allows selectivity of the radical addition onto less reactive internal alkynes to be determined.
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Stork and Baine first demonstrated the synthetic utility of vinyl radicals by carrying out reductive cyclizations to form five and six-membered rings.1 Bu3SnH and azobisisobutyronitrile (AIBN) were used to carry out vinyl radical cyclizations onto indole yielding mainly reduced adducts.2 Later vinyl radical cyclization with aromatic substitution was achieved using a tin-free chain reaction, where displacement of indole-2-sulfonyl substitutent occurs.3 Effective five and sixmembered intramolecular aromatic substitutions of vinyl radicals using Bu3SnH and AIBN were reported by Padwa et al.4 Although, usually the use of the “reductant” Bu3SnH is not conducive with efficient aromatic substitution, where net-loss of H● or “oxidation” occurs.5,6 Where prior to the substitution the vinyl radical is generated via an intermolecular addition onto alkynes,7 yields of aromatic product are traditionally modest.8 The first synthetically viable tandem process containing an intermolecular addition onto alkynes was reported by Santi et al. using Mn(III)-mediated oxidation of benzylmalonates with the subsequent vinyl radical aromatic substitution giving naphthalene derivatives in moderate to good yields.9 Most recently, Zhou and coworkers used a photo-redox catalytic system to synthesize 2-trifluoromethyl quinolines with imidoyl radical addition onto alkyne followed by aromatic substitution,10 and Li and coworkers reported benzothiophenes under oxidative catalytic conditions with an initial sulfanyl radical addition onto but-2-ynedioates.11 The decomposition of Barton esters {O-acyl thiohydroxamate ester or pyridine-2-thione-Noxycarbonyl (PTOC)}12 provides a means of achieving aromatic substitution under mild initiatorfree conditions,13-15 as demonstrated by Barton et al. for intermolecular substitution of nucleophilic alkyl radicals onto heteroaromatic salts.13 Most recently we used Barton esters in radical cyclizations resulting in some high yielding five to seven-membered alkyl and cyclopropyl annulations of indoles and benzimidazoles.14,15 In the present article we report a new use of Barton esters, as precursors for one-pot initiator-free cascade/tandem reactions (Scheme 1). The tandem
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reaction involves intermolecular addition of alkyl radicals onto alkyl propiolates or phenylacetylene followed by intramolecular substitution of vinyl radicals onto indoles. Investigations into the use of disubstituted (internal) alkynes with two different substituents are also presented with substitution products demonstrating the selectivity of the radical addition onto the alkyne.
Scheme 1. One-pot initiator-free tandem reactions with 3-substituted indoles
Conditions: (i) Barton ester formation; HOTT (1.5 equiv.), Et3N (3 equiv.), THF-MeCN, rt, dark, 40 min; (ii) radical generation and tandem reactions; alkyl propiolate (8 equiv.), CSA (4 equiv.), MeCN, reflux, 6 h.
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The two-step one-pot protocol involves initial transformation of indole carboxylic acids (e.g. 1a and 1b) into Barton esters using Garner’s HOTT (S-(1-oxido-2-pyridinyl)-1,1,3,3-tetramethyl thiouronium hexafluorophosphate)16 in the absence of light (Scheme 1). HOTT is a useful coupling reagent for forming labile Barton esters from hindered or difficult carboxylic acids. The alkyne is present in excess (8 equivalents) in order to favor the intermolecular addition with reactions using terminal alkynes only proceeding efficiently by carrying out the radical generation and tandem reactions step under acidic conditions {4 equivalents of camphorsulfonic acid (CSA)}. CSA neutralizes remaining triethylamine from the Barton ester formation step, which would otherwise cause inadvertent dimerization of the alkyne. The one-pot reaction sequence begins with Barton ester thermal dissociation to give a nucleophilic ethyl radical that undergoes addition onto the unsubstituted carbon of the terminal alkyne resulting in a vinyl radical for aromatic substitution. The yields for reactions with indole-3-carbonitrile 1a and indole-3-carbaldehyde 1b with methyl and ethyl propiolate to give 6,7-dihydropyrido[1,2-a]indoles 2a, 2b and 2d are 72-79%, with a smaller isolated yield obtained of adduct 2c (68%) from the reaction of 1a with tert-butyl propiolate. The tandem protocol is insensitive to substituents at the indole-3-position with 3methylindole-1-propanoic
acid
(1c)
giving
10-methyl-6,7-dihydropyrido[1,2-a]indole-9-
carboxylate 2e in 76% yield inferring a non-polar cyclizing radical. An efficient chain for vinyl radical aromatic substitution is indicated since no other indole-adducts were observed.
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Scheme 2. One-pot initiator-free tandem reactions with 3-unsubstituted indole
Though ESR data has suggested α-carboxy and α-phenyl vinylic radicals adopt close to linear πtype resonance stabilized structures,17,18 a bent σ-type structure has been reported.19
The
conjugation of the vinyl radical with the adjacent substituent gives an electrophilic radical, while a nucleophilic or neutral radical would be expected if the σ-type structure is adopted. The isolation of only aromatic substitution product 3a in 72% yield from the reaction of 3-unsubstituted 1indolepropanoic acid 1d with methyl propiolate (Scheme 2) confirmed that the intermediate vinyl radical is not influenced by polar effects. In agreement with the Barton et al. chain proposal,13 it can be inferred that the 2-thiopyridinyl S-radical traps the intermediate cyclized radical (the indol3-yl radical in Scheme 1) prior to elimination of 2-pyridinethiol on rearomatization. In contrast to 3-substituted indoles, aromatic S-radical adduct 3a is formed by oxidation during the reaction (presumably by the presence of adventitious oxygen), as confirmed by analysis of the reaction mixture. A mechanism similar to that proposed by Curran and Keller may be involved where hydrogen atom transfer to oxygen would give HO2●,20 which is used to explain the formation of “oxidized” aromatic substitution products from metal hydride-mediated reactions.20,21
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A seven-membered vinyl radical cyclization would allow access to the 7,8-dihydro-6Hazepino[1,2-a]indole system. Use of 3-formylindole-1-butanoic acid 1f and methyl propiolate allowed isolation of seven-membered adduct 7,8-dihydro-6H-azepino[1,2-a]indole-10-carboxylate 5 albeit in 23% yield with 2,3-dihydro-1H-pyrrolo[1,2-a]indole 6 predominating in 51% yield (Scheme 3). The higher yield for the propyl radical adduct 6 is in accordance with a more favourable five-membered cyclization, where there is also compatible polar effects between the nucleophilic radical and the activated 2-position of indole-3-carbaldehyde. We previously reported a five-membered alkyl radical cyclization occurring in 78% yield via the decomposition of the Barton ester of 3-cyanoindole-1-butanoic acid.14 Thus, it seemed that the tandem reaction via a cyclizing seven-membered vinyl radical would be more favorable using non-activated indole 1e (Scheme 2), however unexpectedly this gave 2,3-dihydro-1H-pyrrolo[1,2-a]indole 4 as the major product in 53% yield with the desired 7,8-dihydro-6H-azepino[1,2-a]indole-10-carboxylate 3b isolated in 21% yield. Therefore, it seems that cyclizations of alkyl like vinyl radicals onto indoles are not significantly influenced by polar effects.
Scheme 3. 7,8-Dihydro-6H-azepino[1-2-a]indole via one-pot initiator-free tandem reactions
The addition of nucleophilic radicals (t-Bu●) onto electron-deficient alkyl propiolates is reported to be about ten times faster than onto phenylacetylene (~ k = 2 x 105 M-1 s-1 for alkyl propiolates in 1,2-epoxypropane in comparison to k = 2.1 x 104 M-1 s-1 for phenylacetylene in isopropanol at 300
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K),7,17 which may explain the higher yield for reaction of indole-3-carbaldehyde 1b with alkyl propiolates (Scheme 1) in comparison with phenylacetylene to give the phenyl analogue 7a in 61% yield with 14% isolated of 2-thiopyridine 8a (Scheme 4). Nevertheless this convenient tandem radical approach represents the first synthesis of 9-substituted 6,7-dihydropyrido[1,2-a]indoles.
Scheme 4. One-pot initiator-free tandem reactions with less reactive alkynes
Yields of 8,9-disubstituted 6,7-dihydropyrido[1,2-a]indoles 7b and 7c (5-23% in Scheme 4) were low from reactions of 3-cyanoindole-1-propanoic acid (1a) with internal alkynes under analogous Barton ester conditions to those used for terminal alkynes. The major product was 1-[2-(pyridin-2ylthio)ethyl]-1H-indole-3-carbonitrile (8b) (in 59-81% yield) indicative of a less competitive chain for the aromatic substitution. The addition of the intermediate ethyl radical onto disubstituted alkynes is expected to be slow, as indicated by literature radical addition rates of nucleophilic radicals (t-Bu●) onto ethyl 3-phenylprop-2-ynoate and methyl but-2-ynoate, which are k = 4.2 x 104 M-1 s-1 and k = 5.2 x 102 M-1 s-1 respectively in 1,2-epoxypropane at 300 K;7,17 factors of 10-1000 times slower than onto alkyl propiolates. The selectivity of the ethyl radical addition onto ethyl 3phenylprop-2-ynoate to give an α-styryl vinylic radical and onto methyl hex-2-ynoate to give an α-
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propyl vinylic radical respectively was confirmed by the X-ray crystal structures of the substitution products (Figures S1 and S2); ethyl 10-cyano-9-phenyl-6,7-dihydropyrido[1,2-a]indole-8carboxylate (7b) and methyl 10-cyano-9-propyl-6,7-dihydropyrido[1,2-a]indole-8-carboxylate (7c). The addition is thus dictated by steric factors in agreement with the ESR observation of α-phenyl vinyl radical formation from the addition of Me3C● onto alkyl 3-phenylprop-2-ynoates.17 The lowest yielding tandem reaction to give adduct 7c emphasizes the steric congestion about methyl hex-2-ynoate. The formation of sulfides 8a and 8b is only observed where the alkyne is less reactive, and can occur by alternative ethyl radical reactions such as combination with the 2thiopyridinyl S-radical or from addition onto the Barton ester13,22 to establish a chain. In summary a radical initiator-free (and metal catalyst-free) tandem approach has allowed access to novel alicyclic [1,2-a] ring-fused indoles from commercially available reactants. The aromatic substitutions revealed that the vinyl-radical is non-polar, as well as, allowing the assessment of alkyl radical reactivity towards alkynes.
EXPERIMENTAL SECTION Materials. Carboxylic acids 1b-1f used are commercially available, although 1a and 1e were readily prepared on gram-scale from indole-3-carbonitrile14 and indole23 respectively by basic alkylation and hydrolysis of methyl esters. HOTT though commercially available, was accessed using the literature procedure,16 which in our hands yielded almost 10 g of HOTT in 65% yield. Solvents were distilled over appropriate drying agents prior to use and all reactions were carried out under a nitrogen atmosphere. Et3N was distilled over CaH2 before use. Monitoring of reactions by Thin Layer Chromatography (TLC) was carried out on aluminum-backed plates coated with silica
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gel (60 F254). Column chromatography was carried out using silica gel 60 (particle size 0.040-0.063 mm). Measurements. IR spectra were obtained using a FT-IR spectrophotometer with ATR accessory. NMR spectra were recorded using a 400 MHz instrument. Chemical shifts are reported relative to Me4Si as internal standard and NMR assignments were supported by DEPT and 1H-13C and 1H-1H NMR correlation 2D spectra. High-resolution mass spectra (HRMS) were measured using an ESI time-of-flight mass spectrometer (TOFMS) in positive mode. The precision of all accurate mass measurements is better than 5 ppm. General procedure for one-pot Barton ester formation and tandem radical reactions. Et3N (0.32 mL, 2.30 mmol) in THF (5.7 mL) was added to a mixture of carboxylic acid 1a-1f (0.76 mmol) and HOTT (0.424 g, 1.14 mmol) in MeCN (1.9 mL). The solution was stirred at room temperature in the absence of light for 40 min. Alkyne (6.08 mmol) in MeCN (40 mL) and containing CSA (0.711 g, 3.06 mmol) for alkyl propiolates was added, and heated under reflux for 6 h. The solution was evaporated, dissolved in CH2Cl2 (10 mL) and washed with H2O (2 x 10 mL). The organic extract evaporated and purified by column chromatography using silica gel as absorbent with a gradient elution of hexanes and Et2O or CH2Cl2. Methyl 10-cyano-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2a) (0.151 g, 79%) off-white solid; mp 125-126 ˚C; Rf 0.62 (Et2O); IR vmax (neat, cm-1) 2952, 2215 (CN), 1724 (C=O), 1623, 1473, 1457, 1439, 1337, 1276, 1198, 1082, 1017; 1H NMR (400 MHz, CDCl3) δ 7.77 (d, J 8.2 Hz, 1H, 1-H), 7.32-7.25 (m, 4H), 4.17 (t, J 7.1 Hz, 2H, 6-CH2), 3.98 (s, 3H, OCH3), 2.83-2.77 (m, 2H, 7-CH2);
13
C NMR (100 MHz, CDCl3) δ 164.4 (C=O), 138.7 (8-CH), 135.2, 128.9, 125.7 (all C),
124.8, 122.4 (2,3-CH), 120.2 (1-CH), 115.8 (C), 109.6 (4-CH), 85.1 (CN), 52.2 (OCH3), 39.0 (6CH2), 24.3 (7-CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C15H13N2O2 253.0977; Found 253.0973.
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Methyl 10-formyl-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2b) (0.140 g, 72%) yellow solid; mp 119-120 ˚C; Rf 0.32 (Et2O); IR vmax (neat, cm-1) 2952, 1724 (C=O), 1623 (C=O), 1473, 1457, 1439, 1337, 1276, 1198, 1082, 1017; 1H NMR (400 MHz, CDCl3) δ 10.13 (s, 1H, CHO), 8.36 (d, J 7.7 Hz, 1H, 1-H), 7.33-7.24 (m, 4H), 4.14 (t, J 7.0 Hz, 2H, 6-CH2), 3.88 (s, 3H, OCH3), 2.76 (q, J 7.0 Hz, 2H, 7-CH2);
13
C NMR (100 MHz, CDCl3) δ 186.0 (CHO), 166.3, (COOMe), 138.6 (8-
CH), 136.7, 136.0, 127.1, 126.2 (all C), 124.5, 123.2 (2,3-CH), 122.5 (1-CH), 113.5 (C), 109.2 (4CH), 52.8 (OCH3), 38.6 (6-CH2), 24.2 (7-CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C15H14NO3 256.0974; Found 256.0971. tert-Butyl 10-cyano-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2c) (0.152 g, 68%) yellow oil; Rf 0.76 (Et2O); IR vmax (neat, cm-1) 2978, 2217 (CN), 1715 (C=O), 1533, 1457, 1428, 1393, 1368, 1287, 1254, 1163, 1080, 1016; 1H NMR (400 MHz, CDCl3) δ 7.79 (d, J 7.8 Hz, 1H, 1-H), 7.347.24 (m, 3H), 7.17 (t, J 4.8 Hz, 1H, 8-H), 4.16 (t, J 7.1 Hz, 2H, 6-CH2), 2.78-2.73 (m, 2H, 7CH2), 1.65 (s, 9H, tBu); 13C NMR (100 MHz, CDCl3) δ 163.1 (C=O), 137.3 (8-CH), 135.7, 135.0, 128.7, 127.5 (all C), 124.5, 122.1 (2,3-CH), 120.1 (1-CH), 115.9 (C), 109.5 (4-CH), 83.4 (CN), 38.8 (6-CH2), 28.0 ((CH3)C), 24.1 (7-CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C18H19N2O2 295.1447; Found 295.1439. Ethyl 10-formyl-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2d) (0.160 g, 78%) brown solid; mp 105-107 ˚C; Rf 0.49 (Et2O); IR vmax (neat, cm-1) 1719 (C=O), 1649 (C=O), 1456, 1436, 1400, 1333, 1271, 1185, 1130, 1085; 1H NMR (400 MHz, CDCl3) δ 10.15 (s, 1H, CHO), 8.39-8.36 (m, 1H, 1-H), 7.33-7.22 (m, 4H), 4.36 (q, J 7.1 Hz, 2H), 4.14 (t, J 7.1 Hz, 2H, 6-CH2), 2.78-2.73 (m, 2H, 7-CH2), 1.35 (t, J 7.1 Hz, 3H, CH3);
13
C NMR (100 MHz, CDCl3) δ 186.1 (CHO), 165.9
(COOEt), 138.4 (8-CH), 136.9, 136.0, 127.4, 126.2 (all C), 124.5, 123.2 (2,3-CH), 122.6 (1-CH), 113.5 (C), 109.2 (4-CH), 62.0 (OCH2), 38.6 (6-CH2), 24.2 (7-CH2), 14.2 (CH3); HRMS (ESI) m/z (M+H)+ C16H16NO3 calcd 270.1130, Found 270.1132.
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Methyl 10-methyl-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2e) (0.139 g, 76%) yellow oil; Rf 0.43 (CH2Cl2); IR vmax (neat, cm-1) 2949, 1726 (C=O), 1464, 1438, 1385, 1334, 1271, 1195, 1179, 1077, 1016; 1H NMR (400 MHz, CDCl3) δ 7.57 (d, J 7.8 Hz, 1H, 1-H), 7.22-7.21 (m, 2H), 7.10-7.05 (m, 1H), 6.79 (t, J 5.0 Hz, 1H, 8-H), 4.07 (t, J 6.6 Hz, 2H, 6-CH2), 3.90 (s, 3H, OCH3), 2.68-2.63 (m, 2H, 7-CH2), 2.30 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3) δ 167.1 (C=O), 135.8 (C), 131.8 (8-CH), 129.0, 127.9, 127.2 (all C), 122.7 (CH), 119.3 (1-CH), 119.0 (CH), 109.1 (C), 108.4 (4-CH), 52.1 (OCH3), 38.7 (6-CH2), 24.8 (7-CH2), 10.0 (CH3); HRMS (ESI) m/z: (M+H)+ Calcd for C15H16NO2 242.1181; Found 242.1178. Methyl 10-(pyridine-2-ylthio)-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (3a) (0.183 g, 72%) yellow oil; Rf 0.45 (Et2O); IR vmax (neat, cm-1) 3052, 2947,2983, 1726 (C=O), 1574, 1559, 1448, 1435, 1417, 1341, 1272, 1194, 1170, 1127, 1070; 1H NMR (400 MHz, CDCl3) δ 8.41-8.39 (m, 1H, pyr-6-H), 7.56 (d, J 8.0 Hz, 1H, 1-H), 7.37-7.28 (m, 3H), 7.15-7.11 (m, 1H), 6.95-6.92 (m, 1H, pyr5-H), 6.79 (t, J 4.9 Hz, 1H, 8-H), 6.75 (d, J 8.0 Hz, 1H, pyr-3-H), 4.22 (t, J 6.8 Hz, 2H, 6-CH2), 3.50 (s, 3H, OCH3), 2.77-2.72 (m, 2H, 7-CH2);
13
C NMR (100 MHz, CDCl3) δ 167.3, (C=O),
162.6 (pyr-2-C), 149.2 (pyr-6-CH), 136.5 (pyr-4-CH), 135.1 (C), 133.1 (8-CH), 130.0, 128.0 (both C), 123.9, 121.1 (both CH), 120.2 (X 2 CH), 119.2 (pyr-5-CH), 109.2 (4-CH), 98.3 (C), 52.1 (OCH3), 39.3 (6-CH2), 24.4 (7-CH2); HRMS (ESI) m/z (M+H)+ C19H17N2O2S calcd 337.1011, Found 337.1003. Methyl 11-(pyridin-2-ylthio)-7,8-dihydro-6H-azepino[1,2-a]indole-10-carboxylate (3b) (56 mg, 21%) yellow oil; Rf 0.55 (Et2O); IR vmax (neat, cm-1) 2949, 1719 (C=O), 1577, 1559, 1449, 1417, 1265, 1126, 1045; 1H NMR (400 MHz, CDCl3) δ 8.39-8.37 (m, 1H, pyr-6-H), 7.57 (d, J 8.0 Hz, 1H, 1-H), 7.41 (d, J 8.5 Hz, 1H, 4-H), 7.36 (t, J 7.0 Hz, 1H, 9-H) 7.33-7.28 (m, 2H), 7.17-7.12 (m, 1H) 6.92-6.89 (m, 1H, pyr-5-H), 6.69 (d, J 8.3 Hz, 1H, pyr-3-H), 4.28 (t, J 6.3 Hz, 2H, 6-CH2), 3.43 (s, 3H, OCH3), 2.35-2.29 (m, 2H), 2.28-2.21 (m, 2H);
13
C NMR (100 MHz, CDCl3) δ 166.4
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(C=O), 162.3 (pyr-2-C), 149.1 (pyr-6-CH), 144.0 (9-CH), 139.3, 136.6 (both C), 136.3 (pyr-4CH), 129.1, 128.4 (both C), 123.1, 120.9 (both CH), 119.9 (X 2 CH), 119.0 (pyr-5-CH), 109.3 (4CH), 99.9 (C), 52.0 (OCH3), 41.9 (6-CH2), 29.7, 25.3 (both CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C20H19N2O2S 351.1167; Found 351.1180. 9-(Pyridin-2-ylthio)-2,3-dihydro-1H-pyrrolo[1,2-a]indole (4) (107 mg, 53%) off-white solid; mp 147-148 ˚C; Rf 0.68 (Et2O); IR vmax (neat, cm-1) 2953, 1574, 1558, 1449, 1417, 1339, 1298, 1229, 1125, 1010; 1H NMR (400 MHz, CDCl3) δ 8.41-8.39 (m, 1H, pyr-6-H), 7.55 (d, J 8.0 Hz, 1H, 8H), 7.33-7.28 (m, 2H), 7.22-7.18 (m, 1H), 7.15-7.11 (m, 1H), 6.92-6.89 (m, 1H, pyr-5-H), 6.70 (d, J 8.3 Hz, 1H, pyr-3-H), 4.19 (t, J 7.1 Hz, 2H, 3-CH2), 3.08 (t, J 7.6 Hz, 2H, 1-CH2) 2.70-2.62 (m, 2H, 2-CH2); 13C NMR (100 MHz, CDCl3) δ 163.3 (pyr-2-C), 151.0 (C), 149.3 (pyr-6-CH), 136.4 (pyr-4-CH), 134.1, 133.3 (both C), 121.5, 120.4, 119.2, 119.1 (all CH), 118.9 (pyr-5-CH), 109.9 (5CH), 90.3 (C), 44.8 (3-CH2), 27.0 (2-CH2), 24.0 (1-CH2); HRMS (ESI) m/z (M+H)+ C16H15N2S calcd 267.0956, Found 267.0963. Methyl 11-formyl-7,8-dihydro-6H-azepino[1,2-a]indole-10-carboxylate (5) (47 mg, 23%) yellow oil; Rf 0.33 (Et2O); IR vmax (neat, cm-1) 2952, 1720 (C=O), 1653 (C=O), 1575, 1518, 1458, 1438, 1393, 1376, 1267, 1246, 1210, 1128, 1072, 1051; 1H NMR (400 MHz, CDCl3) δ 10.03 (s, 1H, CHO), 8.34-8.31 (m, 1H, 1-H), 7.69 (t, J 7.3 Hz, 1H, 9-H), 7.40-7.30 (m, 3H), 4.24 (t, J 6.4 Hz, 2H, 6-CH2), 3.82 (s, 3H, OCH3), 2.33-2.31 (m, 4H); 13C NMR (100 MHz, CDCl3) δ 184.8 (CHO), 165.5 (COOMe), 147.0 (9-CH), 142.1, 136.2, 127.8, 125.8 (all C), 123.9, 123.0 (2,3-CH), 121.7 (1CH), 114.9 (C), 109.2 (4-CH), 52.6 (OCH3), 41.5 (6-CH2), 30.4, 24.7 (both CH2); HRMS (ESI) m/z: (M+H)+ calcd for C16H16NO3 270.1130; Found 270.1136. 2,3-Dihydro-1H-pyrrolo[1,2-a]indole-9-carbaldehyde (6) (72 mg, 51%) off-white solid; mp 134135 °C (mp24 136 °C).
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9-Phenyl-6,7-dihydropyrido[1,2-a]indole-10-carbaldehyde (7a) (0.127 g, 61%) yellow solid; 123124 °C; Rf 0.65 (Et2O); IR vmax (neat, cm-1) 2978, 1643 (C=O), 1576, 1469, 1456, 1428, 1394, 1368, 1331, 1300, 1175, 1127, 1067; 1H NMR (400 MHz, CDCl3) δ 9.20 (s, 1H, CHO), 8.41 (d, J 7.8 Hz, 1H, 1-H), 7.45-7.30 (m, 7H), 7.29-7.26 (m, 1H), 6.33 (t, J 5.0 Hz, 1H, 8-H), 4.22 (t, J 7.1 Hz, 2H, 6-CH2), 2.80-2.74 (m 2H, 7-CH2);
13
C NMR (100 MHz, CDCl3) δ 186.9, (CHO), 142.2,
139.9, 136.2, 134.5 (all C), 129.0 (8-CH, Ph-CH), 128.6, 128.3 (Ph-CH), 126.2 (C), 124.4, 123.2, 123.1 (1,2,3-CH), 113.8 (C), 109.0 (4-CH), 39.4 (6-CH2), 24.0 (7-CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C19H16NO 274.1232; Found 274.1229. 1-[2-(Pyridin-2-ylthio)ethyl]-1H-indole-3-carbaldehyde (8a) (30 mg, 14%) pale yellow oil; Rf 0.35 (Et2O); IR vmax(neat, cm-1) 3046, 2809, 1655 (C=O), 1558, 1577, 1531, 1467, 1454, 1415, 1400, 1388, 1164, 1151, 1125, 1043; 1H NMR (400 MHz, CDCl3) δ 9.95 (s, 1H, CHO), 8.43-8.41 (m, 1H, pyr-6-H), 8.28 (d, J 7.8 Hz, 1H, 4-H), 7.72 (s, 1H, 2-H), 7.56 (d, J 8.3 Hz, 1H, 7-H), 7.487.43 (m, 1H, pyr-4-H), 7.36-7.27 (m, 2H, 5,6-H), 7.14 (d, J 8.2 Hz, 1H, pyr-3-H), 7.02-6.98 (m, 1H, pyr-5-H), 4.48 (t, J 7.1 Hz, 2H, NCH2), 3.55 (t, J 7.1 Hz, 2H, CH2);
13
C NMR (100 MHz,
CDCl3) δ 184.7 (CHO), 156.9 (pyr-2-C), 149.6 (pyr-6-CH) 139.0 (2-CH), 137.2 (C), 136.3 (pyr-4CH), 125.5 (C), 124.1, (pyr-3-CH), 123.0, 122.7 (5,6-CH) 122.2 (4-CH), 120.1 (pyr-5-CH), 118.3 (C), 110.4 (7-CH), 47.1 (NCH2), 29.3 (CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C16H15N2OS 283.0905; Found 283.0911. Ethyl 10-cyano-9-phenyl-6,7-dihydropyrido[1,2-a]indole-8-carboxylate (7b) (60 mg, 23%) offwhite solid; mp 119-120 ˚C; Rf 0.31 (CH2Cl2); IR vmax(neat, cm-1) 2980, 2213 (CN), 1696 (C=O), 1475, 1426, 1378, 1296, 1247, 1216, 1130, 1110, 1017; 1H NMR (400 MHz, CDCl3) δ 7.68 (d, J 8.0 Hz, 1H, 1-H), 7.53-7.43 (m, 3H), 7.40-7.35 (m, 2H), 7.30-7.21 (m, 3H), 4.31 (t, J 7.2 Hz, 2H, 6-CH2), 3.98 (q, J 7.2 Hz, 2H, OCH2), 3.11 (t, J 7.2 Hz, 2H, 7-CH2), 0.90 (t, J 7.2 Hz, 3H, CH3); 13
C NMR (100 MHz, CDCl3) δ 167.3 (C=O), 140.1, 137.5, 135.9, 135.8, 129.2 (all C), 129.1,
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129.0, 128.5 (Ph-CH), 126.6 (C), 125.5, 122.5 (2,3-CH), 120.3 (1-CH), 113.6 (CN), 109.9 (4-CH), 88.3 (C), 61.0 (OCH2), 40.0 (6-CH2), 25.7 (7-CH2), 13.6 (CH3); HRMS (ESI) m/z: (M+H)+ Calcd for C22H19N2O2 343.1447; Found 343.1444. 1-[2-(Pyridin-2-ylthio)ethyl]-1H-indole-3-carbonitrile (8b) (0.125 g, 59%) yellow oil; Rf 0.28 (CH2Cl2); IR vmax (neat, cm-1) 2927, 2217 (CN), 1578, 1558, 1533, 1467, 1455, 1415, 1392, 1349, 1283, 1250, 1167, 1125, 1015; 1H NMR (400 MHz, CDCl3) δ 8.42 (d, J 5.0 Hz, 1H, pyr-6-H), 7.73 (d, J 8.3 Hz, 1H, pyr-4-H), 7.64 (s, 1H, 2-H), 7.61 (d, J 8.2 Hz, 1H, 4-H), 7.52-7.46 (m, 1H, 7-H), 7.37-7.26 (m, 2H, 5,6-H), 7.15 (d, J 7.8 Hz, 1H, pyr-3-H), 7.04-7.00 (m, 1H, pyr-5-H), 4.50 (t, J 7.1 Hz, 2H, NCH2), 3.55 (t, J 7.1 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 156.8 (pyr-2-C), 149.6 (pyr-6-CH), 136.4 (pyr-4-CH), 135.4 (C), 135.3 (2-CH), 128.0 (C), 124.0 (pyr-3-CH) 122.8, 122.3 (5,6-CH), 120.1 (pyr-5-CH), 120.0 (4-CH), 116.0 (CN), 110.9 (7-CH), 85.9 (C), 47.1 (NCH2), 29.4 (CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C16H14N3S 280.0908; Found 280.0905. Methyl 10-cyano-9-propyl-6,7-dihydropyrido[1,2-a]indole-8-carboxylate(7c) (11 mg, 5%) pale yellow solid; mp 120-122 ˚C; Rf 0.63 (Et2O); IR vmax (neat, cm-1) 2925, 2215 (CN), 1715 (C=O), 1596, 1457, 1423, 1292, 1272, 1245, 1213, 1086; 1H NMR (400 MHz, CDCl3) δ 7.77 (d, J 8.0 Hz, 1H, 1-H), 7.39-7.33 (m, 2H), 7.29-7.25 (m, 1H), 4.15 (t, J 7.1 Hz, 2H, 6-CH2), 3.85 (s, 3H, OCH3), 3.18 (t, J 8.0 Hz, 2H), 2.93 (t, J 7.1 Hz, 2H, 7-CH2), 1.76-1.66 (m, 2H), 1.08 (t, J 7.3 Hz, 3H);
13
C
NMR (100 MHz, CDCl3) δ 167.5 (C=O), 149.9, 139.9, 135.4, 128.9 (all C), 125.3, 122.6 (2,3-CH), 122.3 (C), 120.1 (1-CH), 116.3 (CN), 109.9 (4-CH), 85.4 (C), 52.1 (OCH3), 39.9 (6-CH2), 31.2 (CH2), 25.6 (7-CH2), 23.8 (CH2), 13.9 (CH3); HRMS (ESI) m/z: (M+H)+ Calcd for C18H19N2O2 295.1447; Found 295.1445.
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ASSOCIATED INFORMATION Supporting Information X-ray crystal structures of 7b and 7c (with CIF files) and 1H NMR and 13C NMR of all products. Crystallographic data (excluding structure factors) has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC-996100 for (7b) and CCDC996099 for (7c). This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGEMENTS The authors thank the College of Science, National University of Ireland Galway for a Postgraduate Scholarship for Robert Coyle.
REFERENCES 1. Stork, G.; Baine, N. H. J. Am. Chem. Soc. 1982, 104, 2321. 2. Ziegler, F. E.; Jeroncic, L. O. J. Org. Chem. 1991, 56, 3479. 3. Caddick, S.; Shering, C. L.; Wadman, S. N. Tetrahedron 2000, 56, 465. 4. Padwa, A.; Rashatasakhon, P.; Ozdemir, A. D.; Willis, J. J. Org. Chem. 2005, 70, 519. 5. Minisci, F.; Fontana, F.; Pianese, G.; Yan, Y. M. J. Org. Chem. 1993, 58, 4207. 6. Bowman, W. R.; Storey, J. M. D. Chem. Soc. Rev. 2007, 36, 1803. 7. Wille, U. Chem. Rev. 2012, 113, 813. 8. (a) Leardini, R.; Nanni, D.; Pedulli, G. F.; Tundo, A.; Zanardi, G. J. Chem. Soc., Perkin Trans. 1 1986, 1591. (b) Back, T. G.; Krishna, M. V. J. Org. Chem. 1988, 53, 2533. (c) Leardini, R.; Nanni, D.; Tundo, A.; Zanardi, G. Tetrahedron Lett. 1998, 39, 2441. (d) Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1998, 63, 537. (e) Leardini, R.; Nanni, D.; Zanardi, G. J. Org. Chem. 2000, 65, 2763.
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9. Santi, R.; Bergamini, F.; Citterio, A.; Sebastiano, R.; Nicolini, M. J. Org. Chem. 1992, 57, 4250. (b) Citterio, A.; Sebastiano, R.; Maronati, A.; Santi, R.; Bergamini, F. J. Chem. Soc., Chem. Commun. 1994, 1517. 10. Dong, X.; Xu, Y.; Liu, J. J.; Hu, Y.; Xiao, T.; Zhou, L. Chem. –Eur. J. 2013, 19, 16928. 11. Liu, K.; Jia, F.; Xi, H.; Li, Y.; Zheng, X.; Guo, Q.; Shen, B.; Li, Z. Org. Lett. 2013, 15, 2026. 12. (a) Barton, D. H. R.; Crich, D.; Motherwell, W. B. J. Chem. Soc., Chem. Commun. 1983, 939. (b) Barton, D. H. R.; Crich, D.; Motherwell, W. B. Tetrahedron 1985, 41, 3901. 13. Barton, D. H. R.; Garcia, B.; Togo, H.; Zard, S. Z. Tetrahedron Lett. 1986, 27, 1327. 14. Coyle, R.; Fahey, K.; Aldabbagh, F. Org. Biomol. Chem. 2013, 11, 1672. 15. Fahey, K.; Coyle, R.; McArdle, P.; Aldabbagh, F. Arkivoc 2013, (iii), 401. 16. Garner, P.; Anderson, J. T.; Dey, S.; Youngs, W. J.; Galat, K. J. Org. Chem. 1998, 63, 5732. 17. Rubin, H.; Fischer, H. Helv. Chim. Acta. 1996, 79, 1670. 18. Neilson, G. W.; Symons, M. C. R. J. Chem. Soc., Perkin Trans. 2 1973, 1405. 19. (a) Ito, O.; Omori, R.; Matsuda, M. J. Am. Chem. Soc. 1982, 104, 3934. (b) Korth, H.-G.; Lusztyk, J.; Ingold, K. U. J. Chem. Soc., Perkin Trans. 2 1990, 1997. (c) Goumans, T. P. M.; van Alem, K.; Lodder, G. Eur. J. Org. Chem. 2008, 2008, 435. 20. Curran, D. P.; Keller, A. I. J. Am. Chem. Soc. 2006, 128, 13706. 21. Fagan, V.; Bonham, S.; Carty, M. P.; Aldabbagh, F. Org. Biomol. Chem. 2010, 8, 3149. 22. Newcomb, M.; Park, S. U. J. Am. Chem. Soc. 1986, 108, 4132. 23. Guida, W. C.; Mathre, D. J. J. Org. Chem. 1980, 45, 3172. 24. Moody, C. J.; Norton, C. L. J. Chem. Soc., Perkin Trans. 1 1997, 2639.
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Tandem Reactions via Barton Esters with Intermolecular Addition and Vinyl Radical Substitution onto Indole Robert Coyle, Patrick McArdle, and Fawaz Aldabbagh J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo5008543 • Publication Date (Web): 16 May 2014 Downloaded from http://pubs.acs.org on May 26, 2014
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Tandem Reactions via Barton Esters with Intermolecular Addition and Vinyl Radical Substitution onto Indole
Robert Coyle, Patrick McArdle and Fawaz Aldabbagh* School of Chemistry, National University of Ireland Galway, University Road, Galway, Ireland ∗ Corresponding author. Tel.: +353 (0) 91 493120; fax: +353 (0) 91 49557 e-mail: [email protected] (F. Aldabbagh)
ABSTRACT: A one-pot initiator-free Barton ester decomposition, tandem radical addition onto alkyl propiolates or phenylacetylene with aromatic substitution of the resultant vinyl radical allows convenient access to new 9-substituted 6,7-dihydropyrido[1,2-a]indoles. Propyl radical cyclizations compete when forming the expanded 7,8-dihydro-6H-azepino[1,2-a]indole system. 2-Thiopyridinyl S-radical is incorporated into aromatic adducts when using unsubstituted indole-1-alkanoic acid precursors. X-ray crystallography on substitution products allows selectivity of the radical addition onto less reactive internal alkynes to be determined.
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Stork and Baine first demonstrated the synthetic utility of vinyl radicals by carrying out reductive cyclizations to form five and six-membered rings.1 Bu3SnH and azobisisobutyronitrile (AIBN) were used to carry out vinyl radical cyclizations onto indole yielding mainly reduced adducts.2 Later vinyl radical cyclization with aromatic substitution was achieved using a tin-free chain reaction, where displacement of indole-2-sulfonyl substitutent occurs.3 Effective five and sixmembered intramolecular aromatic substitutions of vinyl radicals using Bu3SnH and AIBN were reported by Padwa et al.4 Although, usually the use of the “reductant” Bu3SnH is not conducive with efficient aromatic substitution, where net-loss of H● or “oxidation” occurs.5,6 Where prior to the substitution the vinyl radical is generated via an intermolecular addition onto alkynes,7 yields of aromatic product are traditionally modest.8 The first synthetically viable tandem process containing an intermolecular addition onto alkynes was reported by Santi et al. using Mn(III)-mediated oxidation of benzylmalonates with the subsequent vinyl radical aromatic substitution giving naphthalene derivatives in moderate to good yields.9 Most recently, Zhou and coworkers used a photo-redox catalytic system to synthesize 2-trifluoromethyl quinolines with imidoyl radical addition onto alkyne followed by aromatic substitution,10 and Li and coworkers reported benzothiophenes under oxidative catalytic conditions with an initial sulfanyl radical addition onto but-2-ynedioates.11 The decomposition of Barton esters {O-acyl thiohydroxamate ester or pyridine-2-thione-Noxycarbonyl (PTOC)}12 provides a means of achieving aromatic substitution under mild initiatorfree conditions,13-15 as demonstrated by Barton et al. for intermolecular substitution of nucleophilic alkyl radicals onto heteroaromatic salts.13 Most recently we used Barton esters in radical cyclizations resulting in some high yielding five to seven-membered alkyl and cyclopropyl annulations of indoles and benzimidazoles.14,15 In the present article we report a new use of Barton esters, as precursors for one-pot initiator-free cascade/tandem reactions (Scheme 1). The tandem
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reaction involves intermolecular addition of alkyl radicals onto alkyl propiolates or phenylacetylene followed by intramolecular substitution of vinyl radicals onto indoles. Investigations into the use of disubstituted (internal) alkynes with two different substituents are also presented with substitution products demonstrating the selectivity of the radical addition onto the alkyne.
Scheme 1. One-pot initiator-free tandem reactions with 3-substituted indoles
Conditions: (i) Barton ester formation; HOTT (1.5 equiv.), Et3N (3 equiv.), THF-MeCN, rt, dark, 40 min; (ii) radical generation and tandem reactions; alkyl propiolate (8 equiv.), CSA (4 equiv.), MeCN, reflux, 6 h.
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The two-step one-pot protocol involves initial transformation of indole carboxylic acids (e.g. 1a and 1b) into Barton esters using Garner’s HOTT (S-(1-oxido-2-pyridinyl)-1,1,3,3-tetramethyl thiouronium hexafluorophosphate)16 in the absence of light (Scheme 1). HOTT is a useful coupling reagent for forming labile Barton esters from hindered or difficult carboxylic acids. The alkyne is present in excess (8 equivalents) in order to favor the intermolecular addition with reactions using terminal alkynes only proceeding efficiently by carrying out the radical generation and tandem reactions step under acidic conditions {4 equivalents of camphorsulfonic acid (CSA)}. CSA neutralizes remaining triethylamine from the Barton ester formation step, which would otherwise cause inadvertent dimerization of the alkyne. The one-pot reaction sequence begins with Barton ester thermal dissociation to give a nucleophilic ethyl radical that undergoes addition onto the unsubstituted carbon of the terminal alkyne resulting in a vinyl radical for aromatic substitution. The yields for reactions with indole-3-carbonitrile 1a and indole-3-carbaldehyde 1b with methyl and ethyl propiolate to give 6,7-dihydropyrido[1,2-a]indoles 2a, 2b and 2d are 72-79%, with a smaller isolated yield obtained of adduct 2c (68%) from the reaction of 1a with tert-butyl propiolate. The tandem protocol is insensitive to substituents at the indole-3-position with 3methylindole-1-propanoic
acid
(1c)
giving
10-methyl-6,7-dihydropyrido[1,2-a]indole-9-
carboxylate 2e in 76% yield inferring a non-polar cyclizing radical. An efficient chain for vinyl radical aromatic substitution is indicated since no other indole-adducts were observed.
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Scheme 2. One-pot initiator-free tandem reactions with 3-unsubstituted indole
Though ESR data has suggested α-carboxy and α-phenyl vinylic radicals adopt close to linear πtype resonance stabilized structures,17,18 a bent σ-type structure has been reported.19
The
conjugation of the vinyl radical with the adjacent substituent gives an electrophilic radical, while a nucleophilic or neutral radical would be expected if the σ-type structure is adopted. The isolation of only aromatic substitution product 3a in 72% yield from the reaction of 3-unsubstituted 1indolepropanoic acid 1d with methyl propiolate (Scheme 2) confirmed that the intermediate vinyl radical is not influenced by polar effects. In agreement with the Barton et al. chain proposal,13 it can be inferred that the 2-thiopyridinyl S-radical traps the intermediate cyclized radical (the indol3-yl radical in Scheme 1) prior to elimination of 2-pyridinethiol on rearomatization. In contrast to 3-substituted indoles, aromatic S-radical adduct 3a is formed by oxidation during the reaction (presumably by the presence of adventitious oxygen), as confirmed by analysis of the reaction mixture. A mechanism similar to that proposed by Curran and Keller may be involved where hydrogen atom transfer to oxygen would give HO2●,20 which is used to explain the formation of “oxidized” aromatic substitution products from metal hydride-mediated reactions.20,21
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A seven-membered vinyl radical cyclization would allow access to the 7,8-dihydro-6Hazepino[1,2-a]indole system. Use of 3-formylindole-1-butanoic acid 1f and methyl propiolate allowed isolation of seven-membered adduct 7,8-dihydro-6H-azepino[1,2-a]indole-10-carboxylate 5 albeit in 23% yield with 2,3-dihydro-1H-pyrrolo[1,2-a]indole 6 predominating in 51% yield (Scheme 3). The higher yield for the propyl radical adduct 6 is in accordance with a more favourable five-membered cyclization, where there is also compatible polar effects between the nucleophilic radical and the activated 2-position of indole-3-carbaldehyde. We previously reported a five-membered alkyl radical cyclization occurring in 78% yield via the decomposition of the Barton ester of 3-cyanoindole-1-butanoic acid.14 Thus, it seemed that the tandem reaction via a cyclizing seven-membered vinyl radical would be more favorable using non-activated indole 1e (Scheme 2), however unexpectedly this gave 2,3-dihydro-1H-pyrrolo[1,2-a]indole 4 as the major product in 53% yield with the desired 7,8-dihydro-6H-azepino[1,2-a]indole-10-carboxylate 3b isolated in 21% yield. Therefore, it seems that cyclizations of alkyl like vinyl radicals onto indoles are not significantly influenced by polar effects.
Scheme 3. 7,8-Dihydro-6H-azepino[1-2-a]indole via one-pot initiator-free tandem reactions
The addition of nucleophilic radicals (t-Bu●) onto electron-deficient alkyl propiolates is reported to be about ten times faster than onto phenylacetylene (~ k = 2 x 105 M-1 s-1 for alkyl propiolates in 1,2-epoxypropane in comparison to k = 2.1 x 104 M-1 s-1 for phenylacetylene in isopropanol at 300
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K),7,17 which may explain the higher yield for reaction of indole-3-carbaldehyde 1b with alkyl propiolates (Scheme 1) in comparison with phenylacetylene to give the phenyl analogue 7a in 61% yield with 14% isolated of 2-thiopyridine 8a (Scheme 4). Nevertheless this convenient tandem radical approach represents the first synthesis of 9-substituted 6,7-dihydropyrido[1,2-a]indoles.
Scheme 4. One-pot initiator-free tandem reactions with less reactive alkynes
Yields of 8,9-disubstituted 6,7-dihydropyrido[1,2-a]indoles 7b and 7c (5-23% in Scheme 4) were low from reactions of 3-cyanoindole-1-propanoic acid (1a) with internal alkynes under analogous Barton ester conditions to those used for terminal alkynes. The major product was 1-[2-(pyridin-2ylthio)ethyl]-1H-indole-3-carbonitrile (8b) (in 59-81% yield) indicative of a less competitive chain for the aromatic substitution. The addition of the intermediate ethyl radical onto disubstituted alkynes is expected to be slow, as indicated by literature radical addition rates of nucleophilic radicals (t-Bu●) onto ethyl 3-phenylprop-2-ynoate and methyl but-2-ynoate, which are k = 4.2 x 104 M-1 s-1 and k = 5.2 x 102 M-1 s-1 respectively in 1,2-epoxypropane at 300 K;7,17 factors of 10-1000 times slower than onto alkyl propiolates. The selectivity of the ethyl radical addition onto ethyl 3phenylprop-2-ynoate to give an α-styryl vinylic radical and onto methyl hex-2-ynoate to give an α-
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propyl vinylic radical respectively was confirmed by the X-ray crystal structures of the substitution products (Figures S1 and S2); ethyl 10-cyano-9-phenyl-6,7-dihydropyrido[1,2-a]indole-8carboxylate (7b) and methyl 10-cyano-9-propyl-6,7-dihydropyrido[1,2-a]indole-8-carboxylate (7c). The addition is thus dictated by steric factors in agreement with the ESR observation of α-phenyl vinyl radical formation from the addition of Me3C● onto alkyl 3-phenylprop-2-ynoates.17 The lowest yielding tandem reaction to give adduct 7c emphasizes the steric congestion about methyl hex-2-ynoate. The formation of sulfides 8a and 8b is only observed where the alkyne is less reactive, and can occur by alternative ethyl radical reactions such as combination with the 2thiopyridinyl S-radical or from addition onto the Barton ester13,22 to establish a chain. In summary a radical initiator-free (and metal catalyst-free) tandem approach has allowed access to novel alicyclic [1,2-a] ring-fused indoles from commercially available reactants. The aromatic substitutions revealed that the vinyl-radical is non-polar, as well as, allowing the assessment of alkyl radical reactivity towards alkynes.
EXPERIMENTAL SECTION Materials. Carboxylic acids 1b-1f used are commercially available, although 1a and 1e were readily prepared on gram-scale from indole-3-carbonitrile14 and indole23 respectively by basic alkylation and hydrolysis of methyl esters. HOTT though commercially available, was accessed using the literature procedure,16 which in our hands yielded almost 10 g of HOTT in 65% yield. Solvents were distilled over appropriate drying agents prior to use and all reactions were carried out under a nitrogen atmosphere. Et3N was distilled over CaH2 before use. Monitoring of reactions by Thin Layer Chromatography (TLC) was carried out on aluminum-backed plates coated with silica
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gel (60 F254). Column chromatography was carried out using silica gel 60 (particle size 0.040-0.063 mm). Measurements. IR spectra were obtained using a FT-IR spectrophotometer with ATR accessory. NMR spectra were recorded using a 400 MHz instrument. Chemical shifts are reported relative to Me4Si as internal standard and NMR assignments were supported by DEPT and 1H-13C and 1H-1H NMR correlation 2D spectra. High-resolution mass spectra (HRMS) were measured using an ESI time-of-flight mass spectrometer (TOFMS) in positive mode. The precision of all accurate mass measurements is better than 5 ppm. General procedure for one-pot Barton ester formation and tandem radical reactions. Et3N (0.32 mL, 2.30 mmol) in THF (5.7 mL) was added to a mixture of carboxylic acid 1a-1f (0.76 mmol) and HOTT (0.424 g, 1.14 mmol) in MeCN (1.9 mL). The solution was stirred at room temperature in the absence of light for 40 min. Alkyne (6.08 mmol) in MeCN (40 mL) and containing CSA (0.711 g, 3.06 mmol) for alkyl propiolates was added, and heated under reflux for 6 h. The solution was evaporated, dissolved in CH2Cl2 (10 mL) and washed with H2O (2 x 10 mL). The organic extract evaporated and purified by column chromatography using silica gel as absorbent with a gradient elution of hexanes and Et2O or CH2Cl2. Methyl 10-cyano-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2a) (0.151 g, 79%) off-white solid; mp 125-126 ˚C; Rf 0.62 (Et2O); IR vmax (neat, cm-1) 2952, 2215 (CN), 1724 (C=O), 1623, 1473, 1457, 1439, 1337, 1276, 1198, 1082, 1017; 1H NMR (400 MHz, CDCl3) δ 7.77 (d, J 8.2 Hz, 1H, 1-H), 7.32-7.25 (m, 4H), 4.17 (t, J 7.1 Hz, 2H, 6-CH2), 3.98 (s, 3H, OCH3), 2.83-2.77 (m, 2H, 7-CH2);
13
C NMR (100 MHz, CDCl3) δ 164.4 (C=O), 138.7 (8-CH), 135.2, 128.9, 125.7 (all C),
124.8, 122.4 (2,3-CH), 120.2 (1-CH), 115.8 (C), 109.6 (4-CH), 85.1 (CN), 52.2 (OCH3), 39.0 (6CH2), 24.3 (7-CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C15H13N2O2 253.0977; Found 253.0973.
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Methyl 10-formyl-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2b) (0.140 g, 72%) yellow solid; mp 119-120 ˚C; Rf 0.32 (Et2O); IR vmax (neat, cm-1) 2952, 1724 (C=O), 1623 (C=O), 1473, 1457, 1439, 1337, 1276, 1198, 1082, 1017; 1H NMR (400 MHz, CDCl3) δ 10.13 (s, 1H, CHO), 8.36 (d, J 7.7 Hz, 1H, 1-H), 7.33-7.24 (m, 4H), 4.14 (t, J 7.0 Hz, 2H, 6-CH2), 3.88 (s, 3H, OCH3), 2.76 (q, J 7.0 Hz, 2H, 7-CH2);
13
C NMR (100 MHz, CDCl3) δ 186.0 (CHO), 166.3, (COOMe), 138.6 (8-
CH), 136.7, 136.0, 127.1, 126.2 (all C), 124.5, 123.2 (2,3-CH), 122.5 (1-CH), 113.5 (C), 109.2 (4CH), 52.8 (OCH3), 38.6 (6-CH2), 24.2 (7-CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C15H14NO3 256.0974; Found 256.0971. tert-Butyl 10-cyano-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2c) (0.152 g, 68%) yellow oil; Rf 0.76 (Et2O); IR vmax (neat, cm-1) 2978, 2217 (CN), 1715 (C=O), 1533, 1457, 1428, 1393, 1368, 1287, 1254, 1163, 1080, 1016; 1H NMR (400 MHz, CDCl3) δ 7.79 (d, J 7.8 Hz, 1H, 1-H), 7.347.24 (m, 3H), 7.17 (t, J 4.8 Hz, 1H, 8-H), 4.16 (t, J 7.1 Hz, 2H, 6-CH2), 2.78-2.73 (m, 2H, 7CH2), 1.65 (s, 9H, tBu); 13C NMR (100 MHz, CDCl3) δ 163.1 (C=O), 137.3 (8-CH), 135.7, 135.0, 128.7, 127.5 (all C), 124.5, 122.1 (2,3-CH), 120.1 (1-CH), 115.9 (C), 109.5 (4-CH), 83.4 (CN), 38.8 (6-CH2), 28.0 ((CH3)C), 24.1 (7-CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C18H19N2O2 295.1447; Found 295.1439. Ethyl 10-formyl-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2d) (0.160 g, 78%) brown solid; mp 105-107 ˚C; Rf 0.49 (Et2O); IR vmax (neat, cm-1) 1719 (C=O), 1649 (C=O), 1456, 1436, 1400, 1333, 1271, 1185, 1130, 1085; 1H NMR (400 MHz, CDCl3) δ 10.15 (s, 1H, CHO), 8.39-8.36 (m, 1H, 1-H), 7.33-7.22 (m, 4H), 4.36 (q, J 7.1 Hz, 2H), 4.14 (t, J 7.1 Hz, 2H, 6-CH2), 2.78-2.73 (m, 2H, 7-CH2), 1.35 (t, J 7.1 Hz, 3H, CH3);
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C NMR (100 MHz, CDCl3) δ 186.1 (CHO), 165.9
(COOEt), 138.4 (8-CH), 136.9, 136.0, 127.4, 126.2 (all C), 124.5, 123.2 (2,3-CH), 122.6 (1-CH), 113.5 (C), 109.2 (4-CH), 62.0 (OCH2), 38.6 (6-CH2), 24.2 (7-CH2), 14.2 (CH3); HRMS (ESI) m/z (M+H)+ C16H16NO3 calcd 270.1130, Found 270.1132.
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Methyl 10-methyl-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (2e) (0.139 g, 76%) yellow oil; Rf 0.43 (CH2Cl2); IR vmax (neat, cm-1) 2949, 1726 (C=O), 1464, 1438, 1385, 1334, 1271, 1195, 1179, 1077, 1016; 1H NMR (400 MHz, CDCl3) δ 7.57 (d, J 7.8 Hz, 1H, 1-H), 7.22-7.21 (m, 2H), 7.10-7.05 (m, 1H), 6.79 (t, J 5.0 Hz, 1H, 8-H), 4.07 (t, J 6.6 Hz, 2H, 6-CH2), 3.90 (s, 3H, OCH3), 2.68-2.63 (m, 2H, 7-CH2), 2.30 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3) δ 167.1 (C=O), 135.8 (C), 131.8 (8-CH), 129.0, 127.9, 127.2 (all C), 122.7 (CH), 119.3 (1-CH), 119.0 (CH), 109.1 (C), 108.4 (4-CH), 52.1 (OCH3), 38.7 (6-CH2), 24.8 (7-CH2), 10.0 (CH3); HRMS (ESI) m/z: (M+H)+ Calcd for C15H16NO2 242.1181; Found 242.1178. Methyl 10-(pyridine-2-ylthio)-6,7-dihydropyrido[1,2-a]indole-9-carboxylate (3a) (0.183 g, 72%) yellow oil; Rf 0.45 (Et2O); IR vmax (neat, cm-1) 3052, 2947,2983, 1726 (C=O), 1574, 1559, 1448, 1435, 1417, 1341, 1272, 1194, 1170, 1127, 1070; 1H NMR (400 MHz, CDCl3) δ 8.41-8.39 (m, 1H, pyr-6-H), 7.56 (d, J 8.0 Hz, 1H, 1-H), 7.37-7.28 (m, 3H), 7.15-7.11 (m, 1H), 6.95-6.92 (m, 1H, pyr5-H), 6.79 (t, J 4.9 Hz, 1H, 8-H), 6.75 (d, J 8.0 Hz, 1H, pyr-3-H), 4.22 (t, J 6.8 Hz, 2H, 6-CH2), 3.50 (s, 3H, OCH3), 2.77-2.72 (m, 2H, 7-CH2);
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C NMR (100 MHz, CDCl3) δ 167.3, (C=O),
162.6 (pyr-2-C), 149.2 (pyr-6-CH), 136.5 (pyr-4-CH), 135.1 (C), 133.1 (8-CH), 130.0, 128.0 (both C), 123.9, 121.1 (both CH), 120.2 (X 2 CH), 119.2 (pyr-5-CH), 109.2 (4-CH), 98.3 (C), 52.1 (OCH3), 39.3 (6-CH2), 24.4 (7-CH2); HRMS (ESI) m/z (M+H)+ C19H17N2O2S calcd 337.1011, Found 337.1003. Methyl 11-(pyridin-2-ylthio)-7,8-dihydro-6H-azepino[1,2-a]indole-10-carboxylate (3b) (56 mg, 21%) yellow oil; Rf 0.55 (Et2O); IR vmax (neat, cm-1) 2949, 1719 (C=O), 1577, 1559, 1449, 1417, 1265, 1126, 1045; 1H NMR (400 MHz, CDCl3) δ 8.39-8.37 (m, 1H, pyr-6-H), 7.57 (d, J 8.0 Hz, 1H, 1-H), 7.41 (d, J 8.5 Hz, 1H, 4-H), 7.36 (t, J 7.0 Hz, 1H, 9-H) 7.33-7.28 (m, 2H), 7.17-7.12 (m, 1H) 6.92-6.89 (m, 1H, pyr-5-H), 6.69 (d, J 8.3 Hz, 1H, pyr-3-H), 4.28 (t, J 6.3 Hz, 2H, 6-CH2), 3.43 (s, 3H, OCH3), 2.35-2.29 (m, 2H), 2.28-2.21 (m, 2H);
13
C NMR (100 MHz, CDCl3) δ 166.4
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(C=O), 162.3 (pyr-2-C), 149.1 (pyr-6-CH), 144.0 (9-CH), 139.3, 136.6 (both C), 136.3 (pyr-4CH), 129.1, 128.4 (both C), 123.1, 120.9 (both CH), 119.9 (X 2 CH), 119.0 (pyr-5-CH), 109.3 (4CH), 99.9 (C), 52.0 (OCH3), 41.9 (6-CH2), 29.7, 25.3 (both CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C20H19N2O2S 351.1167; Found 351.1180. 9-(Pyridin-2-ylthio)-2,3-dihydro-1H-pyrrolo[1,2-a]indole (4) (107 mg, 53%) off-white solid; mp 147-148 ˚C; Rf 0.68 (Et2O); IR vmax (neat, cm-1) 2953, 1574, 1558, 1449, 1417, 1339, 1298, 1229, 1125, 1010; 1H NMR (400 MHz, CDCl3) δ 8.41-8.39 (m, 1H, pyr-6-H), 7.55 (d, J 8.0 Hz, 1H, 8H), 7.33-7.28 (m, 2H), 7.22-7.18 (m, 1H), 7.15-7.11 (m, 1H), 6.92-6.89 (m, 1H, pyr-5-H), 6.70 (d, J 8.3 Hz, 1H, pyr-3-H), 4.19 (t, J 7.1 Hz, 2H, 3-CH2), 3.08 (t, J 7.6 Hz, 2H, 1-CH2) 2.70-2.62 (m, 2H, 2-CH2); 13C NMR (100 MHz, CDCl3) δ 163.3 (pyr-2-C), 151.0 (C), 149.3 (pyr-6-CH), 136.4 (pyr-4-CH), 134.1, 133.3 (both C), 121.5, 120.4, 119.2, 119.1 (all CH), 118.9 (pyr-5-CH), 109.9 (5CH), 90.3 (C), 44.8 (3-CH2), 27.0 (2-CH2), 24.0 (1-CH2); HRMS (ESI) m/z (M+H)+ C16H15N2S calcd 267.0956, Found 267.0963. Methyl 11-formyl-7,8-dihydro-6H-azepino[1,2-a]indole-10-carboxylate (5) (47 mg, 23%) yellow oil; Rf 0.33 (Et2O); IR vmax (neat, cm-1) 2952, 1720 (C=O), 1653 (C=O), 1575, 1518, 1458, 1438, 1393, 1376, 1267, 1246, 1210, 1128, 1072, 1051; 1H NMR (400 MHz, CDCl3) δ 10.03 (s, 1H, CHO), 8.34-8.31 (m, 1H, 1-H), 7.69 (t, J 7.3 Hz, 1H, 9-H), 7.40-7.30 (m, 3H), 4.24 (t, J 6.4 Hz, 2H, 6-CH2), 3.82 (s, 3H, OCH3), 2.33-2.31 (m, 4H); 13C NMR (100 MHz, CDCl3) δ 184.8 (CHO), 165.5 (COOMe), 147.0 (9-CH), 142.1, 136.2, 127.8, 125.8 (all C), 123.9, 123.0 (2,3-CH), 121.7 (1CH), 114.9 (C), 109.2 (4-CH), 52.6 (OCH3), 41.5 (6-CH2), 30.4, 24.7 (both CH2); HRMS (ESI) m/z: (M+H)+ calcd for C16H16NO3 270.1130; Found 270.1136. 2,3-Dihydro-1H-pyrrolo[1,2-a]indole-9-carbaldehyde (6) (72 mg, 51%) off-white solid; mp 134135 °C (mp24 136 °C).
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9-Phenyl-6,7-dihydropyrido[1,2-a]indole-10-carbaldehyde (7a) (0.127 g, 61%) yellow solid; 123124 °C; Rf 0.65 (Et2O); IR vmax (neat, cm-1) 2978, 1643 (C=O), 1576, 1469, 1456, 1428, 1394, 1368, 1331, 1300, 1175, 1127, 1067; 1H NMR (400 MHz, CDCl3) δ 9.20 (s, 1H, CHO), 8.41 (d, J 7.8 Hz, 1H, 1-H), 7.45-7.30 (m, 7H), 7.29-7.26 (m, 1H), 6.33 (t, J 5.0 Hz, 1H, 8-H), 4.22 (t, J 7.1 Hz, 2H, 6-CH2), 2.80-2.74 (m 2H, 7-CH2);
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C NMR (100 MHz, CDCl3) δ 186.9, (CHO), 142.2,
139.9, 136.2, 134.5 (all C), 129.0 (8-CH, Ph-CH), 128.6, 128.3 (Ph-CH), 126.2 (C), 124.4, 123.2, 123.1 (1,2,3-CH), 113.8 (C), 109.0 (4-CH), 39.4 (6-CH2), 24.0 (7-CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C19H16NO 274.1232; Found 274.1229. 1-[2-(Pyridin-2-ylthio)ethyl]-1H-indole-3-carbaldehyde (8a) (30 mg, 14%) pale yellow oil; Rf 0.35 (Et2O); IR vmax(neat, cm-1) 3046, 2809, 1655 (C=O), 1558, 1577, 1531, 1467, 1454, 1415, 1400, 1388, 1164, 1151, 1125, 1043; 1H NMR (400 MHz, CDCl3) δ 9.95 (s, 1H, CHO), 8.43-8.41 (m, 1H, pyr-6-H), 8.28 (d, J 7.8 Hz, 1H, 4-H), 7.72 (s, 1H, 2-H), 7.56 (d, J 8.3 Hz, 1H, 7-H), 7.487.43 (m, 1H, pyr-4-H), 7.36-7.27 (m, 2H, 5,6-H), 7.14 (d, J 8.2 Hz, 1H, pyr-3-H), 7.02-6.98 (m, 1H, pyr-5-H), 4.48 (t, J 7.1 Hz, 2H, NCH2), 3.55 (t, J 7.1 Hz, 2H, CH2);
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C NMR (100 MHz,
CDCl3) δ 184.7 (CHO), 156.9 (pyr-2-C), 149.6 (pyr-6-CH) 139.0 (2-CH), 137.2 (C), 136.3 (pyr-4CH), 125.5 (C), 124.1, (pyr-3-CH), 123.0, 122.7 (5,6-CH) 122.2 (4-CH), 120.1 (pyr-5-CH), 118.3 (C), 110.4 (7-CH), 47.1 (NCH2), 29.3 (CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C16H15N2OS 283.0905; Found 283.0911. Ethyl 10-cyano-9-phenyl-6,7-dihydropyrido[1,2-a]indole-8-carboxylate (7b) (60 mg, 23%) offwhite solid; mp 119-120 ˚C; Rf 0.31 (CH2Cl2); IR vmax(neat, cm-1) 2980, 2213 (CN), 1696 (C=O), 1475, 1426, 1378, 1296, 1247, 1216, 1130, 1110, 1017; 1H NMR (400 MHz, CDCl3) δ 7.68 (d, J 8.0 Hz, 1H, 1-H), 7.53-7.43 (m, 3H), 7.40-7.35 (m, 2H), 7.30-7.21 (m, 3H), 4.31 (t, J 7.2 Hz, 2H, 6-CH2), 3.98 (q, J 7.2 Hz, 2H, OCH2), 3.11 (t, J 7.2 Hz, 2H, 7-CH2), 0.90 (t, J 7.2 Hz, 3H, CH3); 13
C NMR (100 MHz, CDCl3) δ 167.3 (C=O), 140.1, 137.5, 135.9, 135.8, 129.2 (all C), 129.1,
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129.0, 128.5 (Ph-CH), 126.6 (C), 125.5, 122.5 (2,3-CH), 120.3 (1-CH), 113.6 (CN), 109.9 (4-CH), 88.3 (C), 61.0 (OCH2), 40.0 (6-CH2), 25.7 (7-CH2), 13.6 (CH3); HRMS (ESI) m/z: (M+H)+ Calcd for C22H19N2O2 343.1447; Found 343.1444. 1-[2-(Pyridin-2-ylthio)ethyl]-1H-indole-3-carbonitrile (8b) (0.125 g, 59%) yellow oil; Rf 0.28 (CH2Cl2); IR vmax (neat, cm-1) 2927, 2217 (CN), 1578, 1558, 1533, 1467, 1455, 1415, 1392, 1349, 1283, 1250, 1167, 1125, 1015; 1H NMR (400 MHz, CDCl3) δ 8.42 (d, J 5.0 Hz, 1H, pyr-6-H), 7.73 (d, J 8.3 Hz, 1H, pyr-4-H), 7.64 (s, 1H, 2-H), 7.61 (d, J 8.2 Hz, 1H, 4-H), 7.52-7.46 (m, 1H, 7-H), 7.37-7.26 (m, 2H, 5,6-H), 7.15 (d, J 7.8 Hz, 1H, pyr-3-H), 7.04-7.00 (m, 1H, pyr-5-H), 4.50 (t, J 7.1 Hz, 2H, NCH2), 3.55 (t, J 7.1 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 156.8 (pyr-2-C), 149.6 (pyr-6-CH), 136.4 (pyr-4-CH), 135.4 (C), 135.3 (2-CH), 128.0 (C), 124.0 (pyr-3-CH) 122.8, 122.3 (5,6-CH), 120.1 (pyr-5-CH), 120.0 (4-CH), 116.0 (CN), 110.9 (7-CH), 85.9 (C), 47.1 (NCH2), 29.4 (CH2); HRMS (ESI) m/z: (M+H)+ Calcd for C16H14N3S 280.0908; Found 280.0905. Methyl 10-cyano-9-propyl-6,7-dihydropyrido[1,2-a]indole-8-carboxylate(7c) (11 mg, 5%) pale yellow solid; mp 120-122 ˚C; Rf 0.63 (Et2O); IR vmax (neat, cm-1) 2925, 2215 (CN), 1715 (C=O), 1596, 1457, 1423, 1292, 1272, 1245, 1213, 1086; 1H NMR (400 MHz, CDCl3) δ 7.77 (d, J 8.0 Hz, 1H, 1-H), 7.39-7.33 (m, 2H), 7.29-7.25 (m, 1H), 4.15 (t, J 7.1 Hz, 2H, 6-CH2), 3.85 (s, 3H, OCH3), 3.18 (t, J 8.0 Hz, 2H), 2.93 (t, J 7.1 Hz, 2H, 7-CH2), 1.76-1.66 (m, 2H), 1.08 (t, J 7.3 Hz, 3H);
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C
NMR (100 MHz, CDCl3) δ 167.5 (C=O), 149.9, 139.9, 135.4, 128.9 (all C), 125.3, 122.6 (2,3-CH), 122.3 (C), 120.1 (1-CH), 116.3 (CN), 109.9 (4-CH), 85.4 (C), 52.1 (OCH3), 39.9 (6-CH2), 31.2 (CH2), 25.6 (7-CH2), 23.8 (CH2), 13.9 (CH3); HRMS (ESI) m/z: (M+H)+ Calcd for C18H19N2O2 295.1447; Found 295.1445.
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ASSOCIATED INFORMATION Supporting Information X-ray crystal structures of 7b and 7c (with CIF files) and 1H NMR and 13C NMR of all products. Crystallographic data (excluding structure factors) has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC-996100 for (7b) and CCDC996099 for (7c). This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGEMENTS The authors thank the College of Science, National University of Ireland Galway for a Postgraduate Scholarship for Robert Coyle.
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